Sequence data are rapidly becoming the characters of choice for reconstructing phylogenies, as well as for distinguishing the evolutionary forces acting at the DNA level. Both kinds of studies are necessary to develop specific models for how DNA sequences are expected to change over time. I am involved in studying micro-evolutionary mechanisms that generate diversity, identifying genes that are useful in the deep phylogeny of vertebrates and using neutral markers to reconstruct the phylogenetic relationships of organisms.
Processes of diversification are affected not only by selection and adaptation but also by population structure and patterns of gene flow. Analysis of genetic variation in small, isolated populations should allow us to distinguish historical and demographic events in the evolution of these populations. This, in turn, should help us understand the processes which lead to higher level changes such as speciation events.
My interest in biology first started with birds so it should be no surprise that I have used these taxa to examine speciation questions. My research has been at both the population and family the level. Collaborative research projects have led to examination of link between genetics and behavioral ecology and genetics and dispersal in some avian taxa.
Some of the projects I'm involved with are 1)Avian Systematics that includes studies of the phylogenetic relationships of the family Accipitridae and, analysis of the "deep" phylogeny of the class Aves, 2) A genetic analysis of dispersal behavior of the Mexican Spotted Owl (Strix occidentalis lucia) among the "Sky Islands" of southern Arizona and 3) genetic analysis of cooperative breeding groups of Harris' Hawks (Parabuteo unicinctus). Additionally, I have been working with a number of zoos to develop molecular methods of answering animal husbandry questions common in zoological garden settings. A more complete description of some of these research projects follows.
The avian family Accipitridae is a large and diverse family composed of approximately 230 species divided into 56 genera. The evolutionary relationships among accipitrid taxa have been examined previously using phenetic and parsimony approaches and a variety of data sets. These studies have resulted in conflicting phylogenies, presumably due to the high level of homoplasy, perhaps the result of convergence on diet. To develop a firm understanding of the relationships among the major species groups (i.e., morphological types) I analyzed approximately 1000 base pairs of DNA sequence from the mitochondrial encoded cytochrome-b gene. Parsimony, distance and maximum likelihood methods were used to explore the phylogenetic relationships among the Accipitridae.
The major findings of the molecular study include support for the polyphyly of the Kite genera and the sister group relationship of the Osprey (Pandion) with accipitrid taxa. Evidence based on branch length analysis suggests one or two periods of rapid morphological diversification.
Osteological characters from 44 genera were analyzed alone, and in concert with molecular data. These data yielded phylogenetic trees that were very similar to those trees produced solely by molecular data suggesting that we are converging on the true phylogeny. Statistical support for the osteological tree, as demonstrated by bootstrap values, was very weak, however, supporting only partially, the clade of old world vultures (Aegypiinae).
Divergence times estimated from branch lengths suggest that the Accipitridae diverged from other diurnal raptors (Falcons) approximately 75 million years ago. Clades representing the major morphological diversity among the Accipitridae diverged about 35 million years ago over a period of approximately 7 million years.
Future directions for this research will concentrate on understanding the divergence within subgroups of the Accipitridae. Of major interest is an examination of the Buteo like genera and the large genus Buteo with an examination of the phylogenetic relationships between the forest Buteos ( e.g., Gray Hawk, Red-Shouldered Hawk) and those Buteos favoring open country. Understanding the phylogeny of this group will help provide an idea of both the behavioral and morphological evolution in this large group of raptors.
The suggested phylogenetic relationship among the orders of birds has been fluid and has changed dramatically in recent years. These "deep" phylogenies have been based on morphological data and the DNA/DNA hybridization data of Sibley and Ahlquist. Further, the position of the root of the avian clade (where the birds are joined to the reptiles and the rest of the tree of life) is not clear. My current research is focused on identifying nuclear gene sequences that are informative of the phylogenetic relationships at this level. These sequences will provide phylogenetic markers for other vertebrate species as well. Once appropriate gene sequences are identified they will be used to estimate phylogenetic relationships among different orders within the avian clade. These new data will provide hypotheses about avian relationships that can be used to examine results of other suggested phylogenies and to provide an indication of character evolution within the avian clade.
We have used molecular techniques to examine the parentage contribution of each of the adults at a nest. These results show that there is both polyandry (a single female fertilized by more than one male) and polygyny (multiple females laying eggs in the same nest). Further analysis has suggested that adult males are not related to each other but adult females at a nest are related. This suggests that females (either sisters or mother/daughters) dispersed together to new territories and my form long term relationships.
Future research is centered on identifying the source of skewness in sex ratios in Harris' hawk populations. This skewness could be the result of differential survival post fledging or could be the result of a pre-hatching skewness. The evolutionary implications for group formation are quite different depending on the source of sex ratio bias.
Phylogenetic Analysis of the Avian family Accipitridae
My dissertation research centered on the phylogenetic relationships among the major species groups (i.e. morphological types) within the family Accipitridae (Hawks, Kites, Eagles and Old World Vultures).
Golden Eagle
(Aquila chrysaetos)Black Shouldered Kite
(Elanus caeruleus)
Griffin (Gyps fulvus ) and
Egyptian (Neophron percnopterus ) Vultures.Ferruginous Hawk
(Buteo regalis )
These four types are loose groups based on external morphologies. There is considerable evidence that, in some cases, what appears to be a cohesive group is in fact polyphyletic (see below).
The photos to the right are of Coragyps atratus (left) a New World vulture and Aegypius monachus (right) an Old World vulture. Despite the morphological similarities, there is much evidence that the new world vultures are more closely related to the Storks than the diurnal birds of prey. Both the old and new world vultures share similar ecologies and diet. Their external morphological similarities have resulted in their being placed in the same order and, by some, in the same family. This demonstrates the confusion that may result from convergent evolution.Deep Phylogeny of the Class Aves:
Avian Social Behavior
I am also interested in the evolution of avian social systems. I, in collaboration with Jim Dawson, have been examining the genetic and behavioral relationships among birds in social groups of cooperatively breeding Harris' hawks (Parabuteo unicinctus). Nesting groups are composed of from two to seven birds that participate in group hunts, predator detection and providing food for the nestlings. Dispersal information suggests that auxiliary adults are unrelated to the dominant pair at the nest. One of the questions is why unrelated individuals will help to raise young that are not related to them.The Mexican Spotted Owl and Southern Arizona "Sky Islands"
There has been much speculation of and discussion about the viability of populations of the Northern Spotted Owl (Strix occidentalis caurina) in an increasingly fragmented habitat. Adding to the debate is the scarcity of data on the ability of Spotted Owls to disperse across stretches of unsuitable habitat. These questions are also important here in the Southwest where the Mexican Spotted Owl (S. o. lucida) exists in small populations restricted to "sky islands" of suitable habitat surrounded by a "sea of desert." In populations that appear to be geographically isolated it is important to be able to estimate the effectiveness of apparent barriers in limiting the exchange of individuals between sub-populations (gene-flow).
The sky Islands of southern Arizona provide the perfect model system for examining the ability of the Spotted Owl to disperse. The mixed conifer habitats at the tops of the mountains have been isolated from each other since the Pleistocene. If desert regions between the sky islands serve as effective barriers to emigration among these islands, then there should be genetic evidence of isolation.
The question population viability should include considerations of both demographic viability and genetic viability. Demographic viability is ensured when populations are large enough that random variation in birth and death rates do not result in the latter consistently exceeding the former. Genetic viability is also ensured by large populations, but the size of the genetic population or deme to which a group of individuals belongs (e.g., the Mexican Spotted Owls on the mountain refugia of the southwest ranges) is not usually obvious. For this, genetic techniques are needed to quantify the genetic relatedness among multiple populations and to document the degree of gene flow occurring among such populations.
I am working with Russell Duncan and Steve Speich to examine genetic variation of the Mexican Spotted Owl within and among the mountain ranges of southern Arizona. Information gathered from this research should provide data to aid in the development of strategies for Mexican Spotted Owl conservation as well as providing information that may help in understanding the extent that habitat corridors are needed to maintain gene flow among Spotted Owls in the Northwest.
I have been consulting with zoological gardens in an attempt to develop molecular methods to answer animal husbandry questions not answerable using other means.
One such project is trying to identify molecular approaches for sexing penguins. Determining the sex of penguins is difficult as they are monomorphic (both sexes look the same) and their high fat content makes visualization of the gonads by laproscopy difficult. I have been identifying W chromosome (female limited chromosome) specific markers that will allow sex determination from just a small sample of blood.
Other projects have included the identification of hybrid individuals in mixed species displays, paternity testing in marine mammal populations and the analysis of population subdivision in bottle-nosed dolphins.